Insulin receptor and insulin-like growth factors (I and II) receptors structure studies have been extended in rat brain. The binding of labeled peptides to thin sections of frozen fresh rat brain was visualized with autoradiography. By several criteria including structure-activity relationship analysis, these brain peptides receptors were qualitatively indistinguishable from peptide receptors previously characterized on brain and other more typical target tissues and distinct from each other. Each peptide exhibits its own distinctive binding pattern, i.e., each peptide binds to specific cytoarchitectonic structures. Chicken embryos are a suitable model for studying the role of insulin, IGF-I and IGF II and their receptors in embryogenesis. Multiple chick embryo tissues exhibit insulin and IGF-I binding. We have studied the tissue-specific structure differences and the developmental regulation in brain, liver, muscle, heart and limb buds. We have demonstrated that the insulin receptors are active since anti-insulin receptor antibodies cause morphological and biochemical perturbance of development.